Method for the manufacture of rods of meltable material



J. GOORISSEN 3,092,462

METHOD FOR THE MANUFACTURE OF RODS OF MELTABLE MATERIAL June 4, 1963 Filed Jan. 16, 1961 I l I I I r FIG.2

FIG. 1

INVENTOR JAN GOORISSEN BY I M k AGE -r United States Patent 3,092,462 METHOD FOR TIE MANUFACTURE OF RODS 0F MELTABLE MATERKAL Jan Goorissen, Emmasingei, Eindhoven, Netherlands, as-

signor to North American Philips Company, Inc, New York, N.Y., a corporation of Delaware Filed Jan. 16, 1961, Ser. No. 82,747 Claims priority, application Netherlands Jan. 28, 1960 Claims. (Cl. 23-2235) This invention relates to methods for the manufacture of rods of meltable material, for example semi-conductive material, by zone-melting in an atmosphere at least one constituent of which enters into reaction with the molten material while forming a surface film. In such a known method, for example, a floating zone-melting treatment of a rod-shaped body of silicon was carried out in an atmosphere of oxygen, thus producing a film of silicon oxide on the free surface of the molten zone. Such a film intensifies the action of the surface tension of the melt and reduces the possibility of the molten material flowing away. In addition, such a film may assist in inhibiting the absorption of unwanted impurities from the ambiance.

It has been found that by this known method, even when using a monocrystalline seed crystal, polycrystalline material is always obtained, which may be attributed to the occurrence of irregularities in the crystal growth upon solidification of the meltable material due to crystal nucleation at the surface film. An object of the invention is inter alia to manufacture monocrystalline bodies with the use of such a surface film. It is based upon the recognition that the film has a very irregular structure especially at the area where produced first and thus gives rise to disturbances in the crystal lattice of the rod, whereas the portions of the film which have grown afterwards are of a much more regular structure and less liable to cause irregularities.

According to the invention, in a rod of the melta'ble material present in the above-mentioned atmosphere, one extremity of which consists of monocrystalline material, a molten zone is formed in the other extremity of the rod, which zone is advanced towards the first-mentioned monocrystalline extremity of the rod, whereupon a molten zone is moved backwards from the monocrystalline extremity to the other extremity. During the advancement the surface film is formed over substantially the full length of the body, while afterwards during the movement backwards the monocrystalline extremity grows, the molten zone being first of all in contact with those portions of the surface film which have a regular structure.

A risk of the formation of irregularities does not occur until the irregular portion of the surface film is reached, and hence until substantially the whole monocrystalline rod has been formed, due to growth on the monocrystalline end which serves during this process as a seed crystal.

Between the advancing movement and the movement backwards the material is preferably maintained in the molten state so that these movements can succeed one another continuously.

In order to reduce the risk of deformation of the rod upon displacing the zone, preferably an annular zone is used in the advancing movement. The term annular zone is to be understood to mean herein a zone formed by the melting of material at the surface of the rod surrounding a core of solid material. It has been found that the use of an annular zone adds to the regularity of the structure of the surface film. If, in addition, the material is maintained in the molten state between the advancing movement and the movement backwards, the advancing of the annular zone is preferably continued 2 over at least a portion of the direction of length of the monocrystalline end, whereupon the direction of displacement is reversed and, subsequently, during the movement backwards the zone is gradually deepened so that the core also melts.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing showing several stages of the manufacture of a monocrystalline silicon rod by means of a floating zone-melting process in an atmosphere of oxygen.

FIGURE 1 shows diagrammatically in vertical section a device for floating zone-melting, wherein an annular zone in a silicon rod is advanced from the upper end of the rod downwards.

FIGURE 2 shows the same device in vertical section in which at a further stage a molten zone extending over the full diameter of the rod is displaced backwards from the monocrystalline lower end of the rod in the upward direction.

The device shown diagrammatically in FIGURES 1 and 2 comprises a quartz glass tube 1 closed at each end and having a gas inlet 2 and a gas outlet 3, a high frequency coil 4 displaceable in a vertical direction, and two holders 5 and 6 for the material to be treated.

A vertically-arranged rod 7, made of silicon, is secured at its upper end to the upper holder 5 and at its lower end to the lower holder 6. The lower end 8 of the rod 7 consists of monocrystalline silicon and the balance of the rod of polycrystalline silicon.

A flow of pure oxygen at atmospheric pressure is introduced into the tube 1 through the gas inlet 2 and leaves the tube through the outlet 3. The flow-rate of the oxygen may for example be about half a liter per minute. The high-frequency coil 4 is positioned at the same level with the upper end of the silicon rod 7 and subsequently energized so that an annular molten zone 9 is formed in the upper end of the silicon rod (see FIGURE 1). Due to the reaction of the oxygen upon the molten silicon of the zone 9 and upon the heated solid silicon near the molten zone, a surface film 10 of silicon oxide ensues at the free surface of the melt. This surface film is shown with exaggerated thickness for the sake of clarity. By gradually moving downwards the high-frequency coil 4 (indicated by an arrow in FIGURE 1), for instance at a rate of 2 mms. per minute, the zone 9 is advanced downwards, the surface film 10 thus gradually growing in the downward direction. The portion 11 of the surface film 10, which has been formed first, has a very irregular structure. A portion 12 of the surface film 10, which lies below the portion 11 and which is formed as a growth of the portion 11 of the film during the advancement of the zone 9, has in contrast to the film portion 11 produced first, a comparatively regular structure and thickness.

When the zone 9 has arrived at the monocrystalline lower end 8, the movement of the coil 4 is continued for some time so that the molten zone also advances over a portion of the length of the lower end 8. Then the direction of movement of the coil is reversed, so that the coil now moves upwards (indicated by an arrow in FIGURE 2), for instance at a rate of 2 mms. per minute, and the high-frequency current through the coil 4 is slowly raised, the annular zone during the upward movement backwards thus being gradually deepened until the solid core inside the molten ring has completely disappeared and the zone has acquired the shape shown in FIGURE 2. This zone 13 must have acquired its full depth before having reached the initial boundary between the monocrystalline terminal portion 8 and the polycrystalline material which lies above it. The boundary between the material of the end 8 which has not melted and the zone which is gradually widening is indicated by a broken line 14 in FiGURE 2. The growth 15 on the monocrystalline lower end 8 has been found to be also monocrystalline up to the upper end of the rod, the crystal lattice of the lower end 8 being continued in the growth 15.

Although more particularly the treatment of silicon by floating zone-melting in an atmosphere of oxygen has been described by Way of example, it will be evident that the invention is also applicable to the zone-melting treatment of other materials and/or to the use of an atmosphere of a difierent composition.

So many materials, for instance most metals and semiconductors, may in the molten state react with oxygen forming insoluble oxide layers. Some metals, such as titanium, may in the molten state react for instance with nitrogen forming nitrides, such as titanium nitride. The invention is neither limited to floating zone-melting treatments and may, for example, also be applied to zonemelting treatment in an elongated crucible. In the lastmentioned case, instead of using an annular zone in the advancing movement, it is possible to use a zone which extends only from the free surface of the material up to a certain depth above the base of the crucible.

- It is to be noted that the use of a surface film may also be important to inhibit any evaporation of the meltable material or constituents thereof.

hat is claimed is:

1. A method for growing by zone-melting a monocrystalline elongated body of meltable material in an atmosphere containing a constituent reactable with the body material to form a coating of the reaction product on exposed surfaces of the body, comprising providing an elongated body of the material having at one end a monocrystalline portion, providing around the body an atmosphere containing the said constituent, forming at the other end of the body a molten zone extending to the exposed body surfaces, advancing the molten zone through the body toward the said one end to form the coating by reaction between the melt and the reactable constituent whereby the first-formed coating portion tends to be irregular but gradually as the coating extends tends to become more regular, and then advancing a molten zone through the body in the opopsite direction away from the said one end to grow a single crystal portion onward from the monocrystalline portion serving as a seed therefor and commencing at a point where the coating is more regular. I

2. A method for growing by zone-melting a monocryst-alline elongated body of semiconductive material in an atmosphere containing a constituent reactable with the body material to form a coating of the reaction product on exposed surfaces of the body, comprising providing an elongated body of the semi-conductive material having at one end a monocrystalline portion, providing an atmosphere containing the said constituent around the body, forming at the other end of the body a molten zone extending to the exposed body surfaces, advancing the molten zone through the body toward the said one end to form the coating by reaction between the melt and the reactable constituent whereby the first- .formed coating portion tends to be irregular but gradually as the coating extends tends to become more regular, and, when the said molten zone reaches the monocrystalline portion, causing it to extend completely across the body and then advancing it through the body in the opposite direction away from the said one end to grow a single crystal onward from the monocrystalline portion serving as a seed therefor and commencing at a point where the coating is more regular.

3. A method as set forth in claim 2 wherein the first molten zone formed at the other end is annular and extends along the surface of the body.

4. A method for growing by zone-melting a monocrystalline elongated body of silicon semiconductive material in an oxidizing gaseous atmosphere reactable with the silicon to form a silicon oxide coating on exposed surfaces of the body, comprising providing an elongated body of polycrystalline silicon having at one end a monocrystalline seed, providing the oxidizing atmosphere surrounding the body, first forming a molten zone at the other end of the body extending to the exposed body surfaces, advancing the said molten zone through the body toward the said seed end to form the said coating by reaction between the melt and the oxidizing atmos- .phere whereby the first-formed coating portion tends to be irregular but gradually as the coating extends tends to become more regular, and, when the molten zone reaches the seed end, causing it gradually to extend completely across the seed while advancing it through the body in the opposite direction away from the said seed end to grow a singlecrystal commencing at a point where the coating is more regular.

5. A method as set forth in claim 4 wherein the body is vertical, and the zone-melting is a floating zone-melting treatment.

References ited in the tile of this patent UNITED STATES PATENTS 2,789,039 Jensen Apr. 16, 1957 2,897,329 Matare July 28, 1959 2,994,411 Pfann Sept. 15, 1959 2,905,798 Freutel Sept. 22, 1959 2,907,642 Rummel Oct. 6, 1959 2,967,115 Herrick Ian. 3, 1961 FOREIGN PATENTS 745,698 Great Britain Feb. 29, 1956 

1. A METHOD FOR GROWING BY ZONE-MELTING A MONOCRYSTALLINE ELONGATED BODY OF MELTABLE MATERIAL IN AN ATMOSPHERE CONTAINING A CONSTITUENT REACTBLE WITH THE BODY MATERIAL TO FORM A COATING OF THE REACTION PRODUCT ON EXPOSED SURFACES OF THE BODY, COMPRISING PROVIDING AN ELONGATED BODY OF THE MATERIAL HAVING AT ONE END A MONOCRYSTALLINE PORTION, PROVIDING AROUND THE BODY AN ATMOSPHERE CONTAINING THE SAID CONSTITUENT, FORMING AT THE OTHER END OF THE BODY A MOLTEN ZONE EXTENDING TO THE EXPOSED BODY SURFACES, ADVANCING THE MOLTEN ZONE THROUGH THE BODY TOWARD THE SAID ONE END TO FROM THE COATING BY REACTION BETWEEN THE MELT AND THE REACTABLE CONSTITUENT WHEREBY THE FIRST-FORMED COATING PORTION TENDS TO BE IRREGULAR BUT GRADUALLY AS THE COATING EXTENDS TENDS TO BECOME MORE REGULAR, AND THEN ADVANCING A MOLTEN ZONE THROUGH THE BODY IN THE OPPOSITE DIRECTION AWAY FROM THE SAID ONE END TO GROW A SINGLE CRYSTAL PORTION ONWARD FROM THE MONOCRYSTALLINE PORTION SERVING AS A SEED THEREFOR AND COMMENCING AT A POINT WHERE THE COATING IS MORE REGULAR. 